1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of a patient's body organs of interest. In particular, the present invention relates to a novel detector configuration for single photon imaging including single photon emission computed tomography (SPECT) and planar imaging.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Single photon imaging, either planar or SPECT, relies on the use of a collimator placed between the source and a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus inferring the line on which the gamma emission is assumed to have occurred. Single photon imaging techniques require gamma ray detectors that calculate and store both the position of the detected gamma ray and its energy.
Two principal types of collimators have been used in nuclear medical imaging. The predominant type of collimation is the parallel-hole collimator. This type of collimator contains hundreds of parallel holes drilled or etched into a very dense material such as lead. The parallel-hole collimator accepts only photons traveling perpendicular to the scintillator surface, and produces a planar image of the same size as the source object. In general, the resolution of the parallel-hole collimator increases as the holes are made smaller in diameter and longer in length. The parallel-hole collimator offers greater sensitivity than a pinhole collimator, and its sensitivity does not depend on how closely centered the object is to the detector.
The conventional pinhole collimator typically is cone-shaped and has a single small hole drilled in the center of the collimator material. The pinhole collimator generates a magnified image of an object in accordance with its acceptance angle, and is primarily used in studying small organs such as the thyroid or localized objects such as a joint. The pinhole collimator must be placed at a very small distance from the object being imaged in order to achieve acceptable image quality. The pinhole collimator offers the benefit of high magnification of a single object, but loses resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole.
Other known types of collimators include converging and diverging collimators. The converging collimator has holes that are not parallel; rather, the holes are focused toward the organ with the focal point being located in the center of the FOV. The image appears larger at the face of the scintillator using a converging collimator. The converging collimator has a lower sensitivity than the parallel-hole collimator, especially with thick objects.
The diverging collimator results by reversing the direction of the converging collimator. The diverging collimator is typically used to enlarge the FOV, such as would be necessary with a portable camera having a small scintillator. The diverging collimator has a lower sensitivity than the parallel-hole collimator, especially with thick objects.
The ability to image “hot spots” (i.e., small, isolated intense sources of radioactivity) has become an important imaging task in nuclear medicine. Conventional collimated nuclear medicine imaging is not designed to image small, isolated volumes of radioactivity with high resolution or in an efficient manner. It is merely intended to allow CT-like accumulation of planar or projection image data for reconstruction of large body volumes, such as the torso or the pelvis. This imaging task limits the acquisition techniques in nuclear medicine to the parallel-hole and, with corrections for distortions, converging collimation. Consequently, the choice of collimation represents a trade-off between the size of the FOV and the sensitivity and spatial resolution required to properly visualize the target object or organ. Thus, there exists a need in the art for improvements in collimator technology to enhance the imaging of small, isolated intense sources of radioactivity through improved detection efficiency and spatial information.